I'd take a wild guess and say if the guy has $4m to spare then he isn't going to need the car for the weekly trip for groceries, so I expect it'll stay as it is. With that sort of cash behind you I think I'd be tempted to get another mustang and make a mint replica of the car at the start of the movie, then put them side by side on show !
I'm sure I read an article about the car a while ago about how they did the stunts, etc, and the car looked "in need of enthusiast" then. If it hadn't been driven by Steve McQueen in that iconic movie then it would have been crushed long ago. Funny old world, isn't it ?
Brian